In an effort to devise novel treatments for craniofacial birth defects, disease, and injuries, more research needs to be done to understand developmental mechanisms that control jaw length. The jaws often display a range of size-related anomalies including mandibular hypoplasia, retrognathia, asymmetry, and clefting. Our study will provide critical information to address this unmet need by focusing on how jaw length gets regulated during the allocation, proliferation, differentiation, and growth of jaw precursor cells. We employ a unique in vivo strategy to manipulate jaw precursors, which arise embryonically from neural crest mesenchyme (NCM). Our published and preliminary data demonstrate that NCM autonomously executes molecular and histological programs that establish the size and shape of the jaw skeleton. How NCM accomplishes such a complex task, and what specific mechanisms function as determinants of jaw length, remain unknown. Many genetic and embryological studies point to the Sonic Hedgehog (SHH), Fibroblast Growth Factor (FGF), Bone Morphogenetic Protein (BMP), and Transforming Growth Factor-Beta (TGF) pathways as crucial players in establishing jaw length. We hypothesize that NCM differentially regulates and responds to SHH, FGF, BMP, and TGF signaling, in a species-specific manner, which modulates the proliferation, differentiation, and growth of jaw progenitors, and generates variation in jaw length. To test our hypothesis, we combine the species-specific developmental programs of quail and duck in a novel chimeric system. Quail have short jaws whereas those of duck are relatively long, and quail embryos develop much faster than do duck. Exchanging NCM between quail and duck provides a unique way to manipulate signaling between donor NCM and adjacent host tissues, and allows discovery of NCM-dependent processes. We propose three complementary Specific Aims. In Aim 1 we will identify mechanisms that regulate the size of the jaw progenitor population and assess the extent to which they govern species-specific jaw length. We will focus on NCM-mediated cell cycle length, and perform gain- and loss-of-function experiments to resolve when and where changes to progenitor numbers and cell cycle length can affect jaw length. In Aim 2 we will ascertain mechanisms that integrate NCM-mediated SHH and FGF signaling, proliferation dynamics, and jaw length. We will perform gain- and loss-of-function experiments that manipulate epithelial-NCM interactions, and determine when and where changes to SHH and FGF signaling can account for variation in jaw length. In Aim 3 we will determine mechanisms that link NCM- mediated BMP and TGF signaling, differentiation, and bone growth. We will employ gain- and loss-of-function strategies to understand how changes in bone deposition and resorption can affect jaw length. Each Specific Aim is clinically relevant and can serve as a proof-of-principle that molecular therapies can be used to manipulate jaw length. We are confident that our research will provide a foundation for biologically-based, non- surgical methods to treat disorders of the human jaw, whereas currently invasive surgery is the only option.